Ground-based beamforming for satellite communications systems

ABSTRACT

Methods, systems and apparatus for ground-based beamforming of a satellite communications payload ( 200 ) within a satellite communications network ( 100 ). An embodiment of the invention comprises a satellite ( 11 ) communicatively coupled to at least one gateway ( 12 ) via a feeder link ( 13 ) and a plurality of user terminals ( 16 ), each communicatively coupled with the satellite by a user link ( 17 ) where a ground based beam forming system ( 400 ) receives, via feeder link ( 13 ), return path signals ( 452 ) traveling from the user terminals ( 16 ) via the satellite ( 11 ) to the at least one gateway ( 12 ) and forward path signals ( 457 ) traveling from the at least one gateway ( 12 ) via the satellite ( 11 ) to the user terminals ( 16 ), and measures and corrects amplitude and phase errors of the return path signals ( 452 ) and the forward path signals ( 457 ).

RELATED APPLICATION DATA

This application is a continuation of and claims priority under section35 U.S.C. 120 to U.S. patent application Ser. No. 11/467,490, entitledGROUND-BASED BEAMFORMING FOR SATELLITE COMMUNICATIONS SYSTEMS filed onAug. 25, 2006, the entire disclosure of which is incorporated herein byreference for all purposes.

TECHNICAL FIELD

This invention pertains to the field of satellite communicationsnetworks, and, in particular, to forming satellite beams from elementaryfeeds using largely ground-based apparatus and methods.

BACKGROUND ART

Many satellite communications systems require multiple beams to beplaced over a geographic area. FIG. 3, for example, illustrates apattern of coverage to provide service to the United States from ageostationary satellite located at 91 degrees west longitude. Numerousnarrow beams may be formed from a relatively few elementary feeds by aprocess known as beamforming and described, for example, in U.S. Pat.Nos. 5,115,248 and 5,784,030. FIG. 3, for example, shows a pattern of135 spot beams created from a feed array having only 48 elements.Adaptive beamforming permits electrical reconfiguration of the directionof each spot beam, or the formation of beams with different sizes andshapes, each accomplished without the need to change any hardwareelement.

A beamforming capability provides important benefits to many satellitepayloads. For example, it permits a given satellite to operate from anumber of different orbital locations. Thus, a satellite fleet operatorlicensed to operate geostationary spacecraft at multiple orbitallocations may use a common hardware design for all locations andelectrically configure the beam as required to tailor the spot beampattern based on the satellite's location. Moreover, beamforming allowsa satellite, which typically has a fifteen year life span, to be adaptedon orbit to changing traffic patterns or new applications on the ground.

Beamforming, however, is technically challenging to perform on asatellite, inasmuch as the amplitude and phase relationship of each feedelement within an array must be precisely set and provide for both theforward (gateway to satellite to user) signal path and the return (userto satellite to gateway) signal path. Conventional spacebasedbeamforming techniques include analog and digital beamforming networks(BFN's). Analog BFN's are generally co-located with the feed arraybecause it is otherwise difficult to compensate for losses or electricalpath length variations between the feed apertures and the points ofapplication of the beamforming coefficients. Volume and thermalconstraints limit the number of analog BFN's that can be co-located withthe feed array.

Digital BFN's have a better ability to compensate for losses orelectrical path length variations between the feed apertures and thepoints of application of the beamforming coefficients. Accordingly, theycan be employed in the middle of the payload at a considerableelectrical path distance from the feed array, provided that strictattention is paid to design practices minimizing amplitude and phasevariations and calibration processes that accurately track thevariations.

The burdens associated with space-borne BFN's can be substantial, andinclude system reliability degradation, and added hardware mass, cost,power consumption and thermal control requirements. Moreover, if the BFNis on the satellite, the ability to introduce improved technologies andreact flexibly to changing market demand is limited during the life ofthe satellite. Moving BFN functions to the ground is thereforedesirable, but ground-based beamforming systems must overcome severaladditional problems not inherent in space-based beamforming. Among theseare the need to compensate for gateway and satellite componentperformance changes over temperature and life, satellite and groundstation pointing errors, and signal propagation amplitude and phasedispersion effects, including Doppler shifts.

These difficulties have limited the use of ground-based beamformingtechniques. Known prior art techniques apply beamforming in only thereturn direction, or are limited to signals that are code division ortime division multiplexed. Frequency division multiplexing is morecommonly used in space, and offers significant cost and reliabilityadvantages over code division and time division multiplexing.

The present invention provides for ground-based beamforming for both theforward and return communications path. The invention further providesfor ground-based beamforming that can be employed in a system employingfrequency division multiplexed signals.

DISCLOSURE OF INVENTION

Methods, systems and apparatus for ground-based beamforming of asatellite communications payload (200) within a satellite communicationsnetwork (100). An embodiment of the invention comprises a satellite (11)communicatively coupled to at least one gateway (12) via a feeder link(13) and further coupled to a plurality of user terminals (16), eachcommunicatively coupled with the satellite by a user link (17). A groundbased beam forming system (400) measures and corrects amplitude andphase errors of a plurality of return path signals (452) traveling fromthe user terminals (16) via the satellite (11) to at least one gateway(12), and measures and corrects amplitude and phase errors of aplurality of forward path signals (457) traveling from the at least onegateway (12) via the satellite (11) to the user terminals (16).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other more detailed and specific objects and features of thepresent invention are more fully disclosed in the followingspecification, reference being had to the accompanying drawings, inwhich:

FIG. 1 is a system level diagram of an exemplary satellitecommunications network.

FIG. 2 is a block diagram of a satellite communications payload operablewithin the satellite communications network of FIG. 1.

FIG. 3 illustrates an exemplary spot beam pattern suitable for thebeamforming methods and apparatus of the present invention.

FIG. 4 is a block diagram of a ground-based beamforming system inaccordance with the present invention.

FIG. 5 is a process flow diagram illustrating a method for ground-basedbeamforming in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Ground-based beamforming (GBBF) is most advantageous for missions thatrequire spatial re-utilization of communication spectrum (bandwidth)over the satellite field of view (FOV), as exemplified by, but notlimited to, Mobile Satellite Systems (MSS) providing communicationsservices to personal, often hand-held, terminals. Accordingly, FIG. 1illustrates a simplified diagram of an exemplary MSS system 100 to whicha GBBF system is advantageously applied. The MSS system includes asatellite 11, typically though not necessarily located at ageostationary orbital location defined by a longitude. Satellite 11 iscommunicatively coupled to at least one gateway 12 and to a plurality ofuser terminals 16. The user terminals 16 comprise satellite terminalsthat may be handheld mobile telephones or car phones, or may be embeddedin laptop or desktop personal computers, or phone booths. The at leastone gateway 12 is coupled to the public switched telephone network.

Each gateway 12 and the satellite 11 communicate over a feeder link 13,which has both a forward uplink 14 and a return downlink 15. Each userterminal 16 and the satellite 11 communicate over a user link 17 thathas both a forward downlink 18 and a return uplink 19. Pointing beaconstations 101 are optionally employed to provide precise pointingfeedback information to the GBBF system 400 as described hereinafter.

GBBF system 400 is a distributed control system having substantialelements 400 a on the ground, preferably co-located with one gateway 12.These ground-based elements 400 a communicate with pointing beaconstations 101, the co-located gateway 12, and, via the correspondingfeeder link 13, with satellite 11. Certain space-based elements 400 b ofGBBF 400 are necessarily deployed on satellite 11, as discussedhereinafter.

FIG. 2 is a simplified block diagram of a communications payload system200 within satellite 11. The communications payload system 200 has asatellite forward path 201 connecting forward uplink 14 to calibrationnetwork 211 by way of receiver 202 and transmitter 203. Satelliteforward path 201 also typically includes frequency converters,multiplexers, demultiplexers, amplifiers, filters, and other componentsknown in the art but not shown in FIG. 2 for purposes of clarity.Communications payload system 200 also has a satellite return path 207connecting calibration network 211 to return downlink 15 by way ofreceiver 206 and transmitter 205. Again, many individual componentsknown in the art are typically present in satellite return path 207 buthave been omitted from FIG. 2 for purposes of clarity. Satellite forwardpath 201 and satellite return path 207 are communicatively coupledthrough calibration network 211 to forward downlink 18 and return uplink19, respectively, by way of feed array 208 which has multiple feedelements 209.

As discussed in more detail hereinafter, four elements of GBBF 400 areintegrated into satellite communications payload system 200: calibrationnetwork 211, payload beacon 212, payload pilot 213, and tracking masterreference oscillator (MRO) 214. Calibration network 211 includes lowloss couplers that (a) permit user communications traffic to passtransparently in the forward and return directions and (b)simultaneously generate signals having the same amplitude and phasecharacteristics as the user traffic signals at each feed element 209.These signals are passed to satellite return path 207 for transmissionback to the at least one gateway 12. Payload beacon 212 provides anencoded signal of known phase and amplitude to calibration network 211for use in providing forward path signal amplitude and phase errorcorrection, as discussed herebelow. Payload pilot 213 is a signalgenerator for use in providing Doppler frequency shift correction andforward uplink power control as discussed herebelow. Tracking MRO 214 isa tracking master reference oscillator for use in providing Dopplerfrequency shift correction, as discussed herebelow.

FIG. 3 is a pictorial representation of a beam pattern 300 that may beconfigured and controlled by the present invention. In the example shownin FIG. 3, a pattern of 135 spot beams covers the continental UnitedStates, Hawaii, Alaska and Puerto Rico.

FIG. 4 is a block diagram of a GBBF system 400 in accordance with thepresent invention. As indicated in FIG. 4, some space-based elements 400b of GBBF system 400 are necessarily deployed on satellite 11, othersare necessarily disposed on the ground, and still others are preferablyplaced on the ground but may be deployed on satellite 11 withoutdeparting from the teachings of the present invention. The ground-basedelements 400 a of GBBF system 400 are preferably co-located with any onegateway 12.

Some elements of the GBBF system 400 are represented in FIG. 4 ascomputation or signal generating modules, which may be implemented inany combination of hardware, software and firmware. When implemented insoftware, the modules may be implemented in at least one computerreadable medium, such as one or more hard disks, floppy disks, DVD's,CD's, etc.

The GBBF system 400 works cooperatively with certain standardconventional elements of the satellite communications network, forexample, with satellite return path 207 and satellite forward path 201of communications payload system 200. In the preferred embodimentdescribed herein, the spaceborne elements 400 b unique to GBBF system400 are calibration network 201, payload beacon 212, payload pilot 213,and tracking master reference oscillator (MRO) 214.

GBBF system 400 constitutes a beamforming network that controls theoverall shape of beam pattern 300 while in addition computing andapplying beamforming coefficients that compensate for certain errors.Specifically, GBBF system 400 measures and corrects signal amplitude andphase errors associated with satellite return path 207, return downlink15, forward uplink 14, and satellite forward path 201. Further, GBBFsystem 400 controls power of forward uplink 14, minimizes errorsassociated with Doppler frequency shifts in links 13 and 17, andcorrects for satellite 11 pointing error. In a preferred embodiment,signals carried over each feeder link 13 are frequency domainmultiplexed.

Return Path Signal Amplitude and Phase Error Correction.

An encoded return calibration signal 451 having a known amplitude andphase is generated at module 401 and transmitted to the satellitecommunications payload system 200 over each forward uplink 14. Module401 also provides the encoded return calibration signal 451 to amplitudeand phase error calculation module 402. In satellite 11, the encodedreturn calibration signal 451 is processed through satellite forwardpath 201 to calibration network 211. In calibration network 211, returnsignals 452, representing user communications traffic, are tagged withreturn calibration signal 451. The tagged signals 453 are processedthrough the satellite return path 207, and transmitted via returndownlink 15 to amplitude and phase error calculation module 402. Module402 compares the amplitude and phase of the received, tagged signals 453to the amplitude and phase of the calibration signal 451 generated atmodule 401, the difference being representative of the amplitude andphase errors associated with satellite return path 207 and returndownlink 15.

The output of amplitude and phase error calculation module 402 is usedin return beamforming computation module 403 together with the output ofpointing error beamforming module 412 to update the beamformingcoefficients, which are applied to return path signals 452.

Forward Path Signal Amplitude and Phase Error Correction

An encoded forward calibration signal 455, having a known amplitude andphase, is generated by module 404. User signals representing forwardcommunications traffic 454, typically originating in the PSTN and sentvia gateway 12 are tagged with encoded forward calibration signal 455and transmitted to satellite communications payload system 200 overforward uplink 14. In satellite 11, the tagged signals 456 are processedthrough satellite forward path 201, converted to the user frequencyband, and passed through calibration network 211. Calibration network211 outputs signals 457 representing user communications traffic to userterminals 16 via forward downlink 18. Couplers disposed withincalibration network 211 generate signals 458 having the same amplitudeand phase characteristics at each feed element 209 as signals 457.Encoded output 459 of payload beacon generator 212, which is a signalhaving a known phase and amplitude, is passed, along with signals 458,to amplitude and phase error calculation module 405 via satellite returnpath 207 and return downlink 15.

In a preferred embodiment, amplitude and phase error calculation module405 determines a difference between the known amplitude and phase ofencoded forward calibration signal 455 and amplitude and phasecharacteristics of the output signals 458 of the calibration networkcouplers, as received on the ground through return downlink 15. Thisdifference is representative of an error associated with the signal'stotal path including the forward uplink 14, satellite forward path 201,satellite return path 207, and return downlink 15.

Amplitude and phase error calculation module 405 also determines adifference between the known amplitude and phase of encoded payloadbeacon signal 459 and the amplitude and phase characteristics of thepayload beacon signal 459 as received at module 405. This difference isrepresentative of an error associated with the signal's complete returnpath, including the satellite return path 207 and return downlink 15.

Finally, amplitude and phase error calculation module 405 determines theamplitude and phase error associated with the signal's complete forwardpath, including forward uplink 14 and satellite forward path 201, bysubtracting the difference representative of an error associated withthe complete return path from the difference representative of an errorassociated with the total path.

The output of amplitude and phase error calculation module 405 is usedin forward beamforming computation module 406 to update the beamformingcoefficients, which are applied to forward uplink signals 14.

Forward Uplink Power Control

A gateway generated pilot signal 460 is generated by module 408 andtransmitted over forward uplink 14 to satellite communications payload200, which passes gateway generated pilot signal 460 through satelliteforward path 201. Gateway generated pilot signal 460 together with apayload pilot signal 461 generated by payload pilot module 213 is passedover satellite return path 207 to feeder link transmitter 205, whichtransmits transponded gateway generated pilot signal 462 and payloadpilot signal 461 over return downlink 15. Propagation effectscalculation module 409 determines the propagation effect associated withforward uplink 14, by comparing the signal level of the received payloadpilot signal 463 to the signal level of transponded gateway generatedpilot signal 462. This propagation effect is compensated for by uplinkgain adjustment module 410, which provides a control signal 465 thatadjusts the power level of forward uplink 14.

Doppler Frequency Shift Error Minimization

Although for many purposes, geostationary satellites may be consideredmotionless with respect to any point on the ground, they arenevertheless subject to drift velocities that produce noticeable Dopplerfrequency shifts that can prevent accurate ground-based beamforming. Inaccordance with a preferred embodiment of the present invention, errorsassociated with Doppler frequency shifts are minimized in the followingmanner. Gateway generated pilot signal 408 is locked to gateway masterreference oscillator 407 and transmitted over forward uplink 14 tosatellite communications payload 200, where it is applied to satellitetracking master reference oscillator 214. Satellite tracking masterreference oscillator 214 is locked to payload pilot signal 213. Payloadpilot signal 213 is transmitted over return downlink 15 to gatewaytracking master reference oscillator 413, which is locked to thereceived payload pilot signal 464. All frequency conversions on theground are locked to the gateway master reference oscillator 407 in theforward path direction, and locked to the gateway tracking masterreference oscillator 413 in the return path direction. All frequencyconversions on the satellite are locked to the satellite tracking masterreference oscillator 214.

Satellite Pointing Error Correction.

Signals 102 generated by a plurality of pointing beacon stations 101operating at known locations in the user frequency band are received bysatellite 11 over uplinks operating at the same frequency as returnuplink 19, passed through satellite return path 207, and transmitted tothe ground over return downlink 15. Pointing error beamforming module412 calculates and generates pointing error correction coefficients 466to compensate for the error between the measured beam pointing directionand the desired beam pointing direction. Coefficients 466 are providedto forward and return beamforming computation modules 406 and 403,respectively.

The operation of the GBBF system 400 will now be discussed relative tothe flow diagram shown in FIG. 5. At block 501, return calibrationsignal 451 having known amplitude and phase is generated by module 401and passed through the satellite forward path 201 to the calibrationnetwork 211 which also receives ordinary return signals from users 452.At block 504, the user signals 452 are tagged with return calibrationsignal 451 by calibration network 211, and the tagged signals passed,through the satellite return path 207 to block 505. Amplitude and phaseerrors are measured at block 505 by module 402, which compares the knownamplitude and phase of signals 451 with the amplitude and phase ofsignals received from block 504. The errors measured at block 505 areinput to beam forming coefficient calculation, block 530.

At block 511, forward calibration signal 455 is generated by module 404.At block 512 ordinary forward path user signals 454 are tagged with theforward calibration signal 455. The tagged signals 456 are gain adjustedat block 513 by module 410, and passed through satellite forward path201 to calibration network 211. Calibration network 211 also receivesencoded payload beacon signals 459 having a known gain and amplitudefrom payload beacon generator 212. At block 514, amplitude and phasecharacteristics 458 of tagged user signals 456 are output by calibrationnetwork 211 along with payload beacon signals 459. Signals 458 and 459are passed through the satellite return path 207 to block 515. Amplitudeand phase errors are measured at block 515 by module 405, which comparesthe known amplitude and phase of signals 455 and 459 with the amplitudeand phase of signals received from block 514. The errors measured atblock 515 are input to beam forming coefficient calculation, block 530.

Pointing error calculation 520 receives pointing beacon station signals102 and outputs a pointing error estimation to beam forming coefficientcalculation, block 530.

At block 530, beamforming coefficients are calculated by modules 403 and406 for the return and forward paths, respectively.

The above description is included to illustrate the operation of thepreferred embodiments and is not meant to limit the scope of theinvention. The scope of the invention is to be limited only by thefollowing claims. From the above discussion, many variations will beapparent to one skilled in the art that would yet be encompassed by thespirit and scope of the present invention.

1. A satellite communications network having a ground-based beamforming(GBBF) system, said network comprising: a satellite, said satellitecomprising a satellite communications payload, said satellitecommunications payload comprising a satellite return path and asatellite forward path; at least one gateway, communicatively coupledwith the satellite via a feeder link; a plurality of user terminals,each communicatively coupled with the satellite by a user link; aplurality of pointing beacon stations adapted to transmit and receivesignals to and from the satellite; wherein: the satellite communicationspayload comprises a satellite tracking master reference oscillator and acalibration network; and the GBBF system further comprises a masterreference oscillator and a gateway tracking master reference oscillator;and the GBBF system: receives, via the feeder link, (i) a plurality ofreturn path signals traveling from the user terminals via the satelliteto the at least one gateway, and (ii) a plurality of forward pathsignals traveling from the at least one gateway via the satellite to theuser terminals; measures a first set of amplitude and phase errors ofthe plurality of return path signals; corrects said first set ofamplitude and phase errors by generating, on the ground, a first set ofcorrective beam forming coefficients and applying said first set ofcorrective beam forming coefficients to the return path signals;measures a second set of amplitude and phase errors of the plurality offorward path signals; and corrects said second set of amplitude andphase errors by generating, on the ground, a second set of correctivebeam forming coefficients and applying said second set of correctivebeam forming coefficients to the forward path signals; wherein the GBBFsystem corrects amplitude and phase errors of the plurality of returnpath signals by: generating an encoded return calibration signal havinga known amplitude and phase that is received by the satellitecommunications payload and applied to each of a plurality of feedelements in a feed array; receiving from the calibration network overthe satellite return path, and a return downlink between the satelliteand the gateway, a plurality of tagged signals, each of the plurality oftagged signals comprising a combination of user communications trafficand the encoded return calibration signal; determining a differencebetween the known amplitude and phase of the encoded return calibrationsignal and an amplitude and a phase of each of the plurality of taggedsignals, the difference being representative of amplitude and phaseerrors of the plurality of return path signals; and generating andapplying corrective beamforming coefficients to minimize the amplitudeand phase errors of the plurality of return path signals.
 2. Thesatellite communications network of claim 1, wherein the encoded returncalibration signal is applied to each of the plurality of feed elementsin the feed array through the calibration network.
 3. The satellitecommunications network of claim 1, wherein the encoded returncalibration signal is applied to each of the plurality of feed elementsin the feed array via a calibration horn.
 4. The satellitecommunications network of claim 1, wherein the encoded returncalibration signal is encoded by means of a Walsh function and pseudorandom number scrambling.
 5. The satellite communications network ofclaim 1, wherein the encoded return calibration signal is generatedon-board the satellite.
 6. A satellite communications network having aground-based beamforming (GBBF) system, said network comprising: asatellite, said satellite comprising a satellite communications payload,said satellite communications payload comprising a satellite return pathand a satellite forward path; at least one gateway, communicativelycoupled with the satellite via a feeder link; a plurality of userterminals, each communicatively coupled with the satellite by a userlink; a plurality of pointing beacon stations adapted to transmit andreceive signals to and from the satellite; wherein: the satellitecommunications payload comprises a satellite tracking master referenceoscillator and a calibration network; and the GBBF system furthercomprises a master reference oscillator and a gateway tracking masterreference oscillator; and the GBBF system: receives, via the feederlink, (i) a plurality of return path signals traveling from the userterminals via the satellite to the at least one gateway, and (ii) aplurality of forward path signals traveling from the at least onegateway via the satellite to the user terminals; measures a first set ofamplitude and phase errors of the plurality of return path signals;corrects said first set of amplitude and phase errors by generating, onthe ground, a first set of corrective beam forming coefficients andapplying said first set of corrective beam forming coefficients to thereturn path signals; measures a second set of amplitude and phase errorsof the plurality of forward path signals; and corrects said second setof amplitude and phase errors by generating, on the ground, a second setof corrective beam forming coefficients and applying said second set ofcorrective beam forming coefficients to the forward path signals;wherein the GBBF system measures and corrects amplitude and phase errorsof the plurality of forward path signals by: generating an encodedforward calibration signal having a known amplitude and phase and aplurality of tagged signals, each of the plurality of tagged signalscomprising a combination of user communications traffic and the encodedforward calibration signal; transmitting the plurality of tagged signalsover a forward uplink to the communications payload, which passes theplurality of tagged signals through the satellite forward path andthrough the calibration network, which passes the plurality of taggedsignals to a forward downlink, and passes amplitude and phasecharacteristics of the plurality of tagged signals through the satellitereturn path together with an inserted payload beacon signal having aknown amplitude and phase; receiving, over a return downlink, amplitudeand phase characteristics of each of the plurality of tagged signals anda received payload beacon signal; determining a difference between theknown amplitude and phase of the encoded forward calibration signal andamplitude and phase characteristics of each of the plurality of taggedsignals, which difference is representative of an error associated witha total path consisting of the forward uplink, the satellite forwardpath, the satellite return path and the return downlink; determining adifference between the known amplitude and phase of the inserted payloadbeacon signal and an amplitude and phase of the received payload beaconsignal, which difference is representative of an error associated with acomplete return path, the complete return path consisting of thesatellite return path and the return downlink; subtracting thedifference representative of the error associated with the completereturn path from the difference representative of the error associatedwith the total path to obtain an error representative of a completeforward path, the complete forward path consisting of the forward uplinkand the satellite forward path; and generating and applying correctivebeamforming coefficients to minimize amplitude and phase errors of theplurality of forward path signals.
 7. A satellite communications networkhaving a ground-based beamforming (GBBF) system, said networkcomprising: a satellite, said satellite comprising a satellitecommunications payload, said satellite communications payload comprisinga satellite return path and a satellite forward path; at least onegateway, communicatively coupled with the satellite via a feeder link; aplurality of user terminals, each communicatively coupled with thesatellite by a user link; a plurality of pointing beacon stationsadapted to transmit and receive signals to and from the satellite;wherein: the satellite communications payload comprises a satellitetracking master reference oscillator and a calibration network; and theGBBF system further comprises a master reference oscillator and agateway tracking master reference oscillator; and the GBBF system:receives, via the feeder link, (i) a plurality of return path signalstraveling from the user terminals via the satellite to the at least onegateway, and (ii) a plurality of forward path signals traveling from theat least one gateway via the satellite to the user terminals; measures afirst set of amplitude and phase errors of the plurality of return pathsignals; corrects said first set of amplitude and phase errors bygenerating, on the ground, a first set of corrective beam formingcoefficients and applying said first set of corrective beam formingcoefficients to the return path signals; measures a second set ofamplitude and phase errors of the plurality of forward path signals; andcorrects said second set of amplitude and phase errors by generating, onthe ground, a second set of corrective beam forming coefficients andapplying said second set of corrective beam forming coefficients to theforward path signals; wherein the GBBF system controls power of aforward uplink by: transmitting a gateway generated pilot signal overthe forward uplink to the satellite communications payload, which passesthe gateway generated pilot signal through the satellite forward path,passes the gateway generated pilot signal together with a satellitegenerated payload pilot signal over the satellite return path to afeeder link transmitter, and transmits a transponded gateway generatedpilot signal and the satellite generated payload pilot signal to the atleast one gateway over a return downlink; receiving the transpondedgateway generated pilot signal and a received payload pilot signal fromthe return downlink; determining a propagation effect associated withthe forward uplink by comparing a signal level of the received payloadpilot signal to a signal level of the transponded gateway generatedpilot signal; and providing a control signal that adjusts a power levelof the forward uplink to compensate for the propagation effect.
 8. Asatellite communications network having a ground-based beamforming(GBBF) system, said network comprising: a satellite, said satellitecomprising a satellite communications payload, said satellitecommunications payload comprising a satellite return path and asatellite forward path; at least one gateway, communicatively coupledwith the satellite via a feeder link; a plurality of user terminals,each communicatively coupled with the satellite by a user link; aplurality of pointing beacon stations adapted to transmit and receivesignals to and from the satellite; wherein: the satellite communicationspayload comprises a satellite tracking master reference oscillator and acalibration network; and the GBBF system further comprises a masterreference oscillator and a gateway tracking master reference oscillator;and the GBBF system: receives, via the feeder link, (i) a plurality ofreturn path signals traveling from the user terminals via the satelliteto the at least one gateway, and (ii) a plurality of forward pathsignals traveling from the at least one gateway via the satellite to theuser terminals; measures a first set of amplitude and phase errors ofthe plurality of return path signals; corrects said first set ofamplitude and phase errors by generating, on the ground, a first set ofcorrective beam forming coefficients and applying said first set ofcorrective beam forming coefficients to the return path signals;measures a second set of amplitude and phase errors of the plurality offorward path signals; and corrects said second set of amplitude andphase errors by generating, on the ground, a second set of correctivebeam forming coefficients and applying said second set of correctivebeam forming coefficients to the forward path signals; wherein Dopplerfrequency shift errors are minimized by: locking a gateway generatedpilot signal to the master reference oscillator; transmitting thegateway generated pilot signal over a forward uplink to the satellitecommunications payload; applying the gateway generated pilot signal tothe satellite tracking master reference oscillator; locking thesatellite tracking master reference oscillator to a satellite generatedpayload pilot signal; transmitting the satellite generated payload pilotsignal over a return downlink to the at least one gateway; receiving thesatellite generated payload pilot signal at the at least one gateway;and locking the gateway tracking master reference oscillator to thesatellite generated payload pilot signal.
 9. A satellite communicationsnetwork having a ground-based beamforming (GBBF) system, said networkcomprising: a satellite, said satellite comprising a satellitecommunications payload, said satellite communications payload comprisinga satellite return path and a satellite forward path; at least onegateway, communicatively coupled with the satellite via a feeder link; aplurality of user terminals, each communicatively coupled with thesatellite by a user link; a plurality of pointing beacon stationsadapted to transmit and receive signals to and from the satellite;wherein: the satellite communications payload comprises a satellitetracking master reference oscillator and a calibration network; and theGBBF system further comprises a master reference oscillator and agateway tracking master reference oscillator; and the GBBF system:receives, via the feeder link, (i) a plurality of return path signalstraveling from the user terminals via the satellite to the at least onegateway, and (ii) a plurality of forward path signals traveling from theat least one gateway via the satellite to the user terminals; measures afirst set of amplitude and phase errors of the plurality of return pathsignals; corrects said first set of amplitude and phase errors bygenerating, on the ground, a first set of corrective beam formingcoefficients and applying, on the ground, said first set of correctivebeam forming coefficients to the return path signals; measures a secondset of amplitude and phase errors of the plurality of forward pathsignals; and corrects said second set of amplitude and phase errors bygenerating, on the ground, a second set of corrective beam formingcoefficients and applying, on the ground, said second set of correctivebeam forming coefficients to the forward path signals; wherein satellitepointing errors are corrected by: receiving pointing beacon signalsgenerated by the plurality of pointing beacon stations operating atknown locations over a return uplink; transmitting the pointing beaconsignals over a return downlink; and calculating and compensating forerrors between measured beam pointing direction and desired beampointing direction.